Can unvented cylinder work with my old boiler?

I'm assuming you have a combo boiler in your property. If the plumber said, the boiler will drop pressure, that means the boiler's flow rate won't keep up with the hot water demand in your property. Combi boilers in general have limited capacity, they may not deliver water at the pressure you want, particularly if other taps are in use.

Hi Firstly not any plumber can fit an unvented system as he or she must have done there CITB training and a G3 Certification, it dose not mater if they are Gas Safe or a general Plumber they still need it ,and you must ask for proth before you employ them to do the job. if it was me I would not get rid of my boiler unless it was very old and needed updating, but I would keep or update my boiler and still get a unvented system to only feed the shower and possibly the bath as this will give you hot water without having the pressure being lost when you use your shower. You can buy theses unvented system from any good plumbers merchants like STC Plumbers in Ruislip and once you have found a qualified plumber to do the job you can ask him what he would recommend to get. I'm sorry to say I am not qualified to do this as my unvented G3 Certification ran out when I was an engineer with British Gas 10 years ago , but there are plenty of plumbers out there that do have it. Kind Regards Dean

Get a few more quotes for the work No one can give you the answer 100% without seeing the job

I think you don't have enough pressure in the main water! Or your boiler is too old. Even if your boiler loses pressure, you may also have a leak! Even if you have a leak, then the hot water does not work!